A negative-working radiation-sensitive composition is particularly used in a lithographic printing plate precursor, and such a composition generally comprises a radiation-sensitive component, a radical polymerizable component and an initiator system. The development in the field of a lithographic printing plate precursor has recently been directed to a radiation-sensitive composition which enables formation of images by a laser or laser diode, and more specifically enables formation of images and/or development on press. Since the laser can be directly controlled by a computer, laser exposure does not require a silver halide film, like a “mask” with images formed thereon. It is necessary that a high-performance laser or laser diode to be used in a commercially available image setter generally emits radiation having a wavelength of at least 700 nm and also a radiation-sensitive composition is sensitized in a near infrared or infrared range of an electromagnetic spectrum. However, other useful radiation-sensitive compositions are configured to form images under ultraviolet or visible radiation.
A lithographic printing plate precursor includes two types, for example, negative-working and positive-working lithographic printing plate precursors. In the case of a negative-working printing plate precursor, the exposed area in a radiation-sensitive composition is cured and the non-exposed area is removed by dissolving in a developing solution. In contrast, in the case of a positive-working printing plate precursor, the exposed area becomes soluble in a developing solution and removed by dissolving therein, and the non-exposed area remains on a substrate.
Various negative-working radiation-sensitive compositions comprising a reactive polymer binder and lithographic printing plate precursors are known to a person with an ordinary skill in the art. These radiation-sensitive compositions generally contain a radiation-sensitive component, a radical polymerizable component and an initiator system, and also contain various additional components for improving developing properties, photosensitivity and the like.
Various radiation-sensitive compositions in this technical field contain a lot of salts as components thereof. For example, various ionic compounds are used as an IR dye of an infrared radiation absorbing agent, and also various ionic compounds are used as a polymerization initiator. These ionic compounds tend to move and gather in a coating film of a radiation-sensitive composition during storage, and thus leading to localization and further crystallization (such a state is referred to as “blooming”). When a lithographic printing plate precursor, which caused blooming, is exposed so as to form images, the moiety where blooming occurs, undergoes insufficient curing to cause curing unevenness, or even worse, the moiety may sometimes fail to cure. Such a case has been at issue, heretofore.
Patent Literature 1: Japanese Unexamined Patent Publication (Kokai) No. 2009-538446. This publication proposes, as one of methods of preventing blooming of a lithographic printing plate precursor, use of an iodonium borate initiator composition having a specific structure, which does not cause blooming in a coating film of a radiation-sensitive composition. However, it is preferred that blooming can be prevented without using a specific polymerization initiator, infrared radiation absorbing agent and the like.
Blooming decreases shelf life of a lithographic printing plate precursor and makes formation of images unstable. Accordingly, there exits a need for a lithographic printing plate precursor having improved shelf life, which does not cause blooming.